4-port wavelength selective router

ABSTRACT

The present invention relates to a 4-port wavelength selective router and the add/drop multiplexer using the above router. More particularly, it relates to a 4-port wavelength selective router that can effectively routes counter-propagating signals while suppressing multiple reflections generated in the bidirectional transmission systems and networks.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a 4-port wavelength selectiverouter that routes the counter-propagating signals while suppressingrelative intensity noise induced by the multiple back reflections in thebidirectional transmission systems and networks.

[0003] 2. Description of the Related Art

[0004] The bidirectional signal transmission over a single fiber isadvantageous compared with the unidirectional signal transmission. Itenables full-duplex communications between two nodes with a singlestrand of optical fiber. It also alleviates the nonlinear effects ofoptical fiber and thereby enables to achieve higher spectral efficiency.

[0005] However, the bidirectional transmission systems suffer from theoptical back reflections. In bidirectional transmission systems, thecounter-propagating signals are usually allocated at differentwavelengths. Thus, we can suppress the reflected light by using opticalfilters at the receiver. However, we cannot remove the multiplereflected lights in such a way and the multiple reflected lights causesa relative intensity noise. The magnitude of the relative intensitynoise is proportional to the square of optical amplifier gain. Thus therelative intensity noise limits the maximum available amplifier gains ofthe bidirectional transmission systems and networks.

[0006]FIG. 1 shows a schematic diagram of a conventionalwavelength-division multiplexing (WDM) bidirectional transmissionsystem. It also illustrates a relative intensity noise generation pathin the bidirectional transmission system. Each node comprises atransmitter (TX) that generates the WDM signal to be transmitted to theother node and a receiver (RX) that receiving the WDM signalstransmitted from said the other node. Also an optical circulator (Cir)will be installed at each node to route the receiving and thetransmitting signals. Several bidirectional optical amplifiers (BA) areinstalled in the bidirectional transmission link deployed between twonodes to compensate for the loss of optical fibers (10).

[0007] In this WDM bidirectional transmission system, the outputwavelengths of the two nodes are different. We can allocate thewavelengths of the counter-propagating signals according to twodifferent methods: band split scheme and wavelength-interleaved scheme.In the band split bidirectional transmission system as shown in FIG. 2,the wavelengths of WDM signals being transmitted in the same directionare contiguous, while the wavelengths of counter-propagating signals areallocated in different wavelength bands. In the wavelength-interleavedbidirectional transmission systems as shown in FIG. 3, thecounter-propagating signals are interlaid in wavelength domain.

[0008] By allocating the different wavelengths for the optical signalspropagating in the opposite directions, we can eliminate the reflectednoisy light generated by the simple reflection. In other words, even ifthe signal propagating in one direction is reflected at the opticalfibers (10) or other optical components and then combines with the otherdirection signal, the reflected light will be eliminated at the receiver(RX) by an optical filter. However, the optical filter installed at thereceiver (RX) cannot remove the multiple reflected noisy lights becausetheir wavelengths are same as those of the signal lights.

[0009] For an example, a signal reflected at an optical fiber (10) wouldbe amplified at the optical amplifier (BA). If this reflected signalwere to be reflected again at another optical fiber, it would beamplified again, and combined with the original signal as shown inFIG. 1. In such a case, the wavelength of the multiple-reflected noisysignal is identical to that of the original signal, and thus would notbe removed by the optical filter installed at the receiver (RX).Therefore it is necessary a method to suppress the multiple-reflectedlight in a bidirectional optical transmission system.

[0010] In occasion, it is necessary to receive or transmit selectedsignals at an intermediate node of the bidirectional transmission link.In such a case the bidirectional WDM transmission systems furthercomprises add/drop multiplexer (ADM) at the intermediate node. FIG. 4 isa schematic diagram of a WDM bidirectional transmission system furthercomprising a conventional add/drop multiplexer (ADM) that add/dropsignals with specific wavelengths.

[0011] The conventional add/drop multiplexer (ADM) comprises ade-multiplexer (D), 2×2 optical switches (Sw) and a multiplexer (M).

[0012] In this case, two optical circulators (Cir) are used toseparate/combine the counter propagating at the input and the outputport of the add/drop multiplexer (ADM). The optical signals transmittedfrom left to right is first routed to the de-multiplexer by the opticalcirculator (Cir) and then separated as their wavelengths by thede-multiplexer (D). The 2×2 optical switches (Sw) connected to theoutput ports of the de-multiplexer establish transmission paths for thedemultiplexed signals to be dropped or passed though the add/dropmultiplexer (ADM). We can add the same wavelength signals with thedropped signals though the optical switch. The outputs of the opticalswitches are multiplexed by the multiplexer (M) and enter into anotheroptical circulator. The optical circulator route the signals into theoptical fiber.

[0013] Here, the relative intensity noise can be generated through thetransmission path of the signal passing through the ADM as shown in FIG.4.

[0014] Therefore, a means for suppressing the relative intensity noiseshould be incorporated with the with the ADM.

SUMMARY OF THE INVENTION

[0015] The present invention is contrived in order to solve the aboveproblems. It is an object of the present invention to provide a 4-portwavelength selective router that effectively routes thecounter-propagating signals over a single optical fiber whilesuppressing the relative intensity noise induced by the multiple backreflections. The 4-port wavelength selective router in accordance withthe present invention has four ports (1, 2, 3, 4) and three internalsignal transmission paths between port pairs ((1,2), (2,3), (3,4)). The4-port wavelength selective router routes two groups (Group A and GroupB) of signals propagating counter-directionally. The wavelengths of thesignals included in Group A are different from those of the signalsincluded in Group B. When the Group A signals enter at port (2) andoutput through port (3), and Group B signals enter at port (3) andoutput through port (2), the signal transmission characteristics of therouter is characterized as follows; between port (2) and (3) only theGroup A signals can be transmitted from port (2) to port (3), and nosignal can be transmitted oppositely; between port (1) and (2), and port(3) and (4) only the Group B signals or both Group A and B signals canbe transmitted, but both Group A and B signals are not transmittedsimultaneously (in other words, between ports (1) and (2), and ports (3)and (4) at least one port pairs can transmit only the Group B signals);between port (1) and (2), and port (3) and (4) optical signals can betransmitted bidirectionally, but at least one port pair can transmit thesignals from port (1) to (2) or from port (3) to (4); and signalsinputted to a specific port (1, 2, 3, 4) can be transmitted to only oneport (1, 2, 3, 4).

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Exemplary embodiments of the present invention will be describedin conjunction with the drawings in which:

[0017]FIG. 1 shows a schematic diagram for the conventional WDMbidirectional transmission system;

[0018]FIG. 2 and FIG. 3 show the wavelength allocation methods in WDMbidirectional transmission systems and networks;

[0019]FIG. 4 shows a schematic diagram of bidirectional transmissionsystem comprising a conventional add/drop multiplexer (ADM);

[0020]FIG. 5 shows a schematic diagram of the 4-port wavelengthselective router according to the present invention;

[0021]FIG. 6 illustrates the suppression of the multiple reflectionswith the router in FIG. 5;

[0022]FIG. 7 shows a schematic diagram for the 4-port wavelengthselective router according to the first embodiment of the presentinvention;

[0023]FIG. 8, FIG. 9 and FIG. 10 show the propagation path and thepolarization state of the optical wave inputted at the port (1) of therouter in FIG. 7;

[0024]FIG. 11, FIG. 12 and FIG. 13 show the propagation path and thepolarization state of the optical wave inputted through port (2) of therouter in FIG. 7;

[0025]FIG. 14 shows the propagation path of the optical wave inputted atport (3) of the router in FIG. 7;

[0026]FIG. 15 shows the propagation path of the optical wave inputted atport (4) of the router in FIG. 7;

[0027]FIG. 16 and FIG. 17 show the propagation path and the polarizationstate of the optical wave that is reflected at the wavelength selectivefilter (WF);

[0028]FIG. 18 shows the transmission characteristics of the wavelengthselective filter (WF) of FIG. 7 according to an embodiment of thepresent invention;

[0029]FIG. 19 shows the transmission characteristics of the wavelengthselective filter (WF) of FIG. 7 according to another embodiment of thepresent invention;

[0030]FIG. 20 shows a schematic diagram of the 4-port wavelengthselective router according to another embodiment of the presentinvention;

[0031]FIG. 21 and 22 show the transmission characteristics ofwavelength-selective coupler in the router in FIG. 20;

[0032]FIG. 23, 24, 25, and 26 show schematic diagrams of the 4-portwavelength selective router according to other embodiments of thepresent invention;

[0033]FIG. 27 shows a schematic diagram of the bidirectional add/dropmultiplexer (ADM) using the router of the present invention.

EXPLANATIONS FOR MAIN SYMBOLS IN THE DRAWINGS

[0034]4pr: 4-port optical path router, 10: optical fiber, 12: thin film,14: anti reflection (AR) coating, 16 a, 16 b: dielectric coating, BA:bidirectional optical amplifier, BC1,BC2,BC3,BC4: polarization splitter,Cir,Cir1: optical circulator, D: de-multiplexer, Fil1,Fil2: opticalfilter, FR1, FR2: non-reciprocal rotator, M: multiplexer, Re: receivingterminal, RR1,RR2: reciprocal rotator, RX: receiver, Sw: optical switch,TX: transmitter, Tr: transmitting terminal, WF: wavelength selectivefilter, WF1,WF2,WF3: wavelength filter, WSC,WSC1: wavelength selectivecoupler, Iso1,Iso2: optical isolator,

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] The 4-port wavelength selective router (4pr) in accordance withthe present invention is used in order to effectively route two groupsof counter-propagating signals over a single optical fiber and also tosuppress the relative intensity noise induced by multiple backreflections. The above two groups are defined as follows: the firstgroup (Group A) consists of one or more optical signals of differentwavelengths. The second group (Group B) consists of one or more opticalsignals of different wavelengths excluding the signals included in GroupA. Here the methods for dividing the optical signals (Group A, Group B)are above mentioned band split scheme or wavelength-interleaved scheme.

[0036] As shown in FIG. 5, the 4-port wavelength selective router (4pr)has 4 ports (1, 2, 3, 4). Thus the possible combinations of port pairsare 6 in total ((1,2), (2,3), (3,4), (1,3), (2,4), (1,4)). The internalsignals transmission paths exist between port pairs (1) and (2), ports(2) and (3), and ports (3) and (4) Whereas there is no signaltransmission path between ports (1) and (3), ports (2) and (4), andports (1) and (4).

[0037] At port (2) Group A signals are inputted and Group B signals areoutputted. At port (3) Group B signals are inputted and Group A signalsare outputted.

[0038] The signals transmission characteristics of the ports pairs inthe 4-port wavelength selective router (4pr) in accordance with thepresent invention satisfy the following rules. First, between port (2)and (3), only the Group A signals can be transmitted from port (2) toport (3). No signals can be transmitted in the opposite direction.Second, between ports (1) and (2), and ports (3) and (4) the Group Bsignals or both Groups A and B signals are transmitted, but both Group Aand B signals are not transmitted simultaneously between ports (1) and(2), and ports (3) and (4). In other words, between the two port pairs((1,2), (3,4)) at least one port pair should transmit only Group Bsignals. Third, between ports (1) and (2), and ports (3) and (4) opticalwaves are transmitted bidirectionally but at least one port pair shouldtransmit signals only one direction. Namely, between the two port pairs,at least one port pair should transmit signal from port (1) to (2) orport (3) to (4). Fourth, signals inputted to a specific port (1, 2, 3,4) cannot be transmitted to more than one port (1, 2, 3, 4)simultaneously.

[0039] The number of 4-port wavelength selective router (4pr) modulessatisfying the above mentioned rules is eight in total. Table 1 showsthe 4-port wavelength selective router (4pr) module types and theoptical signal transmission characteristics of the ports pairs. Here Xdenotes that no optical wave can pass between the two ports.

[0040] [Table 1]

[0041] The configurable 4-port wavelength selective router (4pr) modulesand signal transmission characteristics of them.

[0042] Examining module #1 of Table 1, the Group B signal can betransmitted from port (1) to port (2), only the Group A signal from port(2) to port (3), and only the Group B signal from port (3) to port (4).Between all other ports and in the other direction, signals are nottransmitted. Continuing to examine module #8 in Table 1, only Group Bsignal can be transmitted from port (1) to port (2) and only the Group Asignal from port (2) to port (3). Between ports (3) and (4), both GroupsA and B signals are transmitted bidirectionally. And no optical wavesare transmitted in other ports pairs and in the other direction.

[0043] As shown in FIG. 6, the 4-port wavelength selective router (4pr)can suppress the multiple reflections generated in the bidirectionaltransmission systems and networks. When ports (1) and (4) are connectedand the counter-propagating signals were inputted/outputted through theports (2) and (3), the signals being inputted at the port (2) cannot betransmitted from port (3) to port (2) and vice versa. The signals beinginputted at the port (3) cannot be transmitted from port (3) to port (1)and vice versa.

[0044]FIG. 7 shows the 4-port wavelength selective router (4pr)according to an embodiment of the present invention. The router (4pr)comprises polarization splitters (BC1, BC2, BC3, BC4), reciprocalpolarization rotators (RR1, RR2), Faraday non-reciprocal polarizationrotators (FR1, FR2), and wavelength selective filter (WF).

[0045] The polarization splitters (BC1, BC4) separates an optical wavewith an arbitrary polarization, which is inputted through each port (1,2, 3, 4), into two optical waves that are polarized perpendicular withrespect to the z-axis (Parallel to the x or y-axis) The abovepolarization splitters also combine two optical polarizedperpendicularly with respect to the z-axis. The polarization splitters(BC2, BC3) cause a displacement in the x-axis. Namely, extraordinarywave having a polarization in x-axis direction is refracted in x-axisdirection at the polarization splitters (BC2, BC3). While ordinary waveshaving a polarization in y-axis direction pass through the polarizationsplitters (BC2, BC3) without any refraction.

[0046] The reciprocal polarization rotators (RR1, RR2) are composed oftwo rotators having opposite rotation directions (PAR; Positive AngleRotator, NAR; Negative Angle Rotator). The two reciprocal polarizationrotators (RR1, RR2) have opposite rotation directions. The combinationof the reciprocal polarization rotators (RR1, RR2) and thenon-reciprocal polarization rotators (FRI, FR2) is a means for thepolarization rotation which makes either two orthogonal optical waveshave parallel polarization or optical waves polarized in parallel haveorthogonal polarizations.

[0047] The wavelength selective filter (WF) is a combination of threefilters (WF1, WF2, WF3). The uppermost of these filters (WF1) in thex-direction, selects the signals passing from port (1) to (2). Thefilter located in the middle (WF2) selects the signals passing fromports (2) to (3), and the filter located on the bottom (WF3) willlikewise the signals passing from ports (3) to (4).

[0048]FIG. 8, FIG. 9 and FIG. 10 show the propagation path and thepolarization state of the optical wave inputted at port (1) observed inx, y, and z directions, respectively. Here the optical wave with anarbitrary polarization would be separated into ordinary andextraordinary waves on the y-z plane by the polarization splitter (BC1).The two orthogonal optical waves will be rotated +45° and −45°,respectively, by the reciprocal polarization rotator (RR1) and thus theywill be polarized in parallel. The above two optical waves will berotated in the same direction by the non-reciprocal polarization rotator(FR1), so that the polarization becomes parallel to the y-axis, and thenpass through the second polarization splitter (BC2) without anydisplacement in x-direction. Therefore, the optical waves pass throughthe filter (WF1) located between the two polarization splitters (BC1,BC2). The optical waves then pass through the polarization splitter(BC3) as ordinary waves, and afterwards pass through the reciprocalpolarization rotator (RR2) and the non-reciprocal polarization rotator(FR2). Here the polarizations of the two optical waves become orthogonaleach other. Thus, the two optical waves are combined at the fourthpolarization splitter (BC4) and then transmitted to port (2).

[0049] Following the identical principle, FIG. 11, FIG. 12 and FIG. 13show the propagation path and polarization state of an optical waveinputted at port (2) observed from the x, y, and z direction,respectively. Similarly with the case of the optical wave inputted atport (1), the optical wave is separated into two orthogonal opticalwaves. They pass through the fourth polarization splitter (BC4), thenon-reciprocal polarization rotator (FR2), and the reciprocalpolarization rotator (RR2). The polarizations of the optical wavesbecome parallel to the x-axis after passing through the reciprocalpolarization rotator (RR2). They pass through the third polarizationsplitter (BC3) as extraordinary waves and will be refracted into the−x-direction. Therefore they pass the filter (WF2) and will be refractedat the second polarization splitter (BC2) in −x-direction. They willthen have orthogonal polarizations each other after passing thenon-reciprocal polarization rotator (FR1) and the reciprocalpolarization rotator (RR1). Finally the optical waves will be combinedat the first polarization splitter (BC1) and be transmitted to port (3).

[0050] The optical wave inputted at port (3) propagates with the samepolarization state in the y-z plane as the optical wave inputted at port(1). However, as illustrated in FIG. 14, it passes the filter (WF3) andis transmitted to port (4).

[0051] The optical wave inputted at port (4) propagates with the samepolarization state in the y-z plane as the optical wave input at port(2) up to the third polarization splitter (BC3). However, as illustratedin FIG. 15, it is refracted at the third polarization splitter (BC3) in−x-direction and cannot be transmitted to any of the other ports (1, 2,3).

[0052] In the above embodiments, the wavelength selective filter (WF)which is composed of the three filters (WF1, WF2, WF3) has the role ofselecting the wavelength of the optical wave which will be transferredbetween the ports (1, 2, 3, 4). Each of the filters (WF1, WF2, WF3)passes the optical waves with specific wavelengths, but reflects orattenuates the optical waves with other wavelengths.

[0053]FIG. 16 and FIG. 17 show the propagation path and the polarizationstate of the optical waves inputted at ports (1) and (2), respectivelyand the propagation path and the polarization state of the optical wavesreflected at the wavelength selective filter (WF).

[0054] The optical wave inputted at port (1) propagates according to thesame transmission paths in FIG. 8 up to wavelength selective filter(WF). After reflected at the filter (WF1) it passes the secondpolarization splitter (BC2) with the identical polarization state as theincident light. But as it passes the non-reciprocal polarization rotator(FR1) and the reciprocal polarization rotator (RR1), the polarizationstate becomes different by 90° with the incident light and then isinputted into the first polarization splitter (BC1). And as shown inFIG. 16 the reflected light does not transmitted into the port (1) or(3).

[0055] The optical wave inputted at port (2) propagates according to thesame transmission paths in FIG. 8 up to wavelength selective filter(WF). After reflected at the filter (WF1) it passes the secondpolarization splitter (BC3) with the identical polarization state as theincident light. But as it passes the reciprocal polarization rotator(RR2) and the non-reciprocal polarization rotator (FR1), thepolarization state becomes different by 90° with the inputted opticalwave and then is inputted into the fourth polarization splitter (BC4).And as shown in FIG. 17 the reflected light is does not transmitted intothe port (2) or (4).

[0056] The length of the filters (WF1, WF2, WF3), in the direction ofthe x-axis, is determined by length of the polarization splitters (BC2,BC3) in the z-axis direction. The optical waves transmitted from port(1) to (2) pass the filter (WF1) in the +z direction. The wavestransmitted from port (2) to (3) pass the filter (WF2) in the −zdirection, and the waves from port (3) to (4) pass the filter (WF3) inthe +z direction. Thus the length of the filter (WF1, WF2, WF3) in thedirection of the x-axis should be adjusted so that it can follow theoptical transmission path according to the length in the direction ofthe z-axis of the second and the third polarization splitter (BC2, BC3).

[0057] We can realize the wavelength selective filter (WF) in numerousmanners and can change depending on the schemes (band split scheme andthe wavelength-interleaved scheme) of the wavelengths allocation.

[0058]FIG. 18 shows the transmission characteristics of the wavelengthselective filter (WF) of the 4-port wavelength selective router (4pr).The wavelength selective filter (WF) is combination of three filters(WF1, WF2, WF3). Each of the filters (WF1, WF2, WF3) can be either aFabry-Perot etalon filter or a comb filter consisting of birefringencecrystals. The filter (WF1) and the filter (WF3) have the identicaltransmission characteristics and have periodic pass/stop-band. Thefilter (WF2) also has periodic pass/stop-band and its period is theidentical to those of the above filter (WF1, WF3), but the passband isshifted by the half of the period from those of the above filters (WF1,WF3).

[0059] The signal transmission characteristics of a 4-port wavelengthselective router (4pr) implemented by using the wavelength selectivefilter described in FIG. 18 is equivalent to the module #1 of Table 1.Here by changing the wavelength selective filter, we can implement othermodules. For example, if filter (WF1) is not used so that all opticalwaves can be transmitted from port (1) to port (2), and the signaltransmission characteristics would be identical to the module #4 ofTable 1. Similarly if the filter (WF3) is not used, the signaltransmission characteristics would be equivalent to the module #6 ofTable 1.

[0060] The wavelength selective filter in FIG. 18 is suitable for thewavelength-interleaved bidirectional transmission systems and networks.We can implement 4-port wavelength selective router (4pr) to be used inband split bidirectional transmission systems and networks by changingthe wavelength selective filter (WF) as shown in FIG. 19. The wavelengthselective filter (WF) can be implement by using a thin film (12) anddielectric coating techniques. One side of the thin film (12) has ananti-reflection (AR) coating (14) while the other side has dielectriccoatings (16 a, 16 b, 16 a). The above dielectric coating (16 a, 16 b,16 a) reflects the signals with specific wavelengths. The filter (WF1)and the filter (WF3) have the identical reflection bands. The dielectriccoatings (16 b) should provides a reflection bands so that thereflection bands of the filter (WF2) should not overlap with those ofthe filter (WF1) and the filter (WF3).

[0061] The optical path router (4pr) according to the present inventioncan also be implemented by combining conventional devices. FIG. 20illustrates an embodiment of the router comprising an optical circulator(Cir) and a wavelength selective coupler (WSC). The optical circulator(Cir) has an input port (a), an output port (c) and a common port (b),and the wavelength selective coupler (WSC) has two input/output ports(d, f) and a common port (e).

[0062] The connections are as follow: The input port (a) of the opticalcirculator (Cir) is connected to port (1) and the common port (b) isconnected to port (2).

[0063] The output port (c) of the circulator (Cir) is connected to oneof the input/output port (d) of the wavelength selective coupler (WSC),and the common input/output (e) of the wavelength selective coupler(WSC) is connected to port (3). The other input/output port (f) of thewavelength selective coupler (WSC) is connected to port (4). In thisembodiment the signal transmission characteristics of the router (4pr)is equivalent to module #5 of Table 1.

[0064] The wavelength selective coupler (WSC) can be divided in to twodifferent types; a wavelength-division multiplexer and awavelength-interleaver. FIG. 21 illustrates the signal transmissioncharacteristics of the wavelength-division multiplexer. Between thecommon port (e) and the input/output port (d), signals within specificwavelength band can be transmitted bidirectionally, while between thecommon port (e) and the other input/output port (f), the signals withinthe other wavelength band excluding the previous one can be transmittedbidirectionally.

[0065] As shown in FIG. 22, the signal transmission characteristics ofthe wavelength-interleaver are as follows. The pass-band between thecommon port (e) and one of the input/output port (d) repeats with aspecific period. Between the common port (e) and the other input/outputport (f), the pass-band period is same but it is shifted by an half ofthe period from that of the common port (e) and the input/output port(d) pair.

[0066] The 4-port wavelength selective router (4pr) shown in FIG. 23replaces the optical circulator (Cir) of FIG. 20 with a wavelengthselective coupler (WSC1) and two optical isolators (Iso1, Iso2). Therouter (4pr) illustrated in FIG. 23 is composed of two wavelengthselective couplers (WSC, WSC1) and two optical isolators (Iso1, Iso2).Each wavelength selective coupler (WSC, WSC1) has two input/output ports((d, f), (d′, f′)) and a common port (e, e′) and each optical isolator(Iso1, Iso2) has one input (g, g′) and one output (h, h′) ports.

[0067] The connections are as follow: The input port (g′) of the opticalisolator (Iso2) is connected to port (1), and the output port (h′) ofthe isolator is connected to one of the input/output port (f′) of thewavelength selective coupler (WSC1). The common port (e′) of thewavelength selective coupler (WSC1) is connected to port (2) and theother input/output port (d′) is connected to the other opticalisolator's (Iso1) input port (g). This isolator's output port (h) isconnected to one of the input/output ports (d) of the other wavelengthselective coupler (WSC). The common port (e) of the wavelength selectivecoupler (WSC) is connected to port (3) and the other input/output port(f) of the wavelength selective coupler (WSC) is connected to port (4).In this embodiment the signal transmission characteristics of the router(4pr) is equivalent to module #3 of Table 1.

[0068]FIG. 24 shows a schematic diagram for the 4-port wavelengthselective router (4pr) according to another embodiment of the presentinvention: The second optical isolator (Iso2) of FIG. 23 is insertedbetween one of the input/output ports (f) of the wavelength selectivecoupler (WSC) and port (4).

[0069] The optical path router (4pr) is composed of two wavelengthselective coupler (WSC, WSC1) having two input/output ports ((d, f),(d′, f′)) and a common port (e, e′), and two optical isolator (Iso1,Iso2) having one input (g, g′) and one output port (h, h′). One of theinput/output port (f′) of the (WSC1) is connected to port (1) and thecommon port (e′) is connected to port (2). The other input/output port(d′) of the wavelength selective coupler (WSC1) is connected to inputport (g) of the optical isolator (Iso1) and output port (h) of theisolator (Iso1) is connected to one of input/output port (d) of thewavelength selective coupler (WSC). The common port (e) of thewavelength selective coupler (WSC) is connected to port (3) and theother input/output port (f) of the wavelength selective coupler (WSC) isconnected to input port (g′) of the isolator (Iso2). The output port(h′) of the isolator (Iso2) is connected to port (4). In this embodimentthe signal transmission characteristics of the router is equivalent tomodule #2 of Table 1.

[0070]FIG. 25 shows a schematic diagram for the 4-port wavelengthselective router (4pr) according to another embodiment of the presentinvention: Two optical isolators (Iso1, Iso2) and the wavelengthselective coupler (WSC) of FIG. 24 are replaced by an optical circulator(Cir1).

[0071] The router (4pr) is composed of a wavelength selective coupler(WSC1) having two input/output ports (d′, f′) and a common port (e′) andan optical circulator (Cir1) with an input (a′), an output (c′), and acommon port (b′). One of the input/output ports (f′) of the wavelengthselective coupler (WSC1) is connected to port (1) and the common port(e′) is connected to port (2). The other input/output port (d′) of thewavelength selective coupler (WSC1) is connected to the input port (a′)of the optical circulator (Cir1) and the common port (b′) of thecirculator (Cir1) is connected to port (3). The output port (c′) of theoptical circulator (Cir1) is connected to port (4). In this embodimentthe signal transmission characteristics of the router is equivalent tomodule #7 of Table 1.

[0072] The wavelength selective coupler (WSC, WSC1) of FIG. 23, FIG. 24,and FIG. 25 is either a wavelength-division multiplexer or awavelength-interleaver like the wavelength selective coupler (WSC) ofFIG. 20.

[0073] The wavelength selective router (4pr) shown in FIG. 26 replacesthe wavelength selective coupler (WSC) of FIG. 20 with an opticalcirculator (Cir1) and two optical filters (Fil1, Fil2). The router (4pr)illustrated in FIG. 26 is composed of two optical circulators (Cir,Cir1) and two optical filters (Fil1, Fil2). Each optical circulator hasan input port (a, a′), an output port (c, c′) and a common port (b, b′),and each optical filter (Fil1, Fil2) has different pass/stop-bands withone input (j, j′) and one output (k, k′) ports.

[0074] The connections are as follow: The input port (a) of the opticalcirculator (Cir) is connected to port (1), and the input/output port (b)of the circulator (Cir) is connected to port (2). The output port (c) ofthe circulator (Cir) is connected to the input port (j) of the opticalfilter (Fill) and the other port (k) of the filter (Fill) is connectedto the input port (a′) of the other circulator (Cir1). This circulator'sinput/output port (b′) is connected to port (3). The output port (c′) ofthe circulator (Cir1) is connected to the input port (j′) of the opticalfilter (Fil2) and the output port (k′) of the filter (Fil2) is connectedto port (4). In this embodiment the signal transmission characteristicsof the router is equivalent to module #4 of Table 1.

[0075] The optical filter (Fil1, Fil2) is a band pass filter whichpasses optical signals within a specific wavelength band while cuttingoff optical signals outside the band, or a comb filter having therepeated pass/stop. And it has the same signal transmissioncharacteristics shown in FIG. 21 or in FIG. 22. The pass and stop-bandsof one optical filter (Fil1) are opposite to those of the other filter(Fil2).

[0076]FIG. 27 illustrates the construction of a bidirectional add/dropmultiplexer (ADM) using the wavelength selective router (4pr) accordingto the present invention. By connecting the conventional add/drop moduleused in unidirectional optical transmissions to port (4) and (1) of theoptical router (4pr), one can implement a bidirectional add/dropmultiplexer (ADM) to add/drop signals propagating from right to left.

[0077] In other words, the input port of the de-multiplexer (D) isconnected to port (4), the output port of the multiplexer (M) to port(1), and 2×2 optical switches (Sw) to the de-multiplexer's output portsand the multiplexer's input ports. Then one can drop or add the signalswith specific wavelengths if the remaining two ports of the opticalswitch are connected to the receiver (RX) and the transmitter (TX),respectively. Unlikely the case of FIG. 4, the 4-port wavelength routerstill suppressed the multiple reflected lights.

[0078] The signals propagating from left to right can also beadded/dropped in the same manners. In this case, we use a symmetricallymodified router (4pr), in which the signal paths for the Group Cpropagating from left to right and for Group D propagating from right toleft are interchanged.

[0079] If the two symmetric routers are connected together, both signalsof Group C (traveling from left to right) and Group D (traveling fromright to left) can be added/dropped at the same time.

[0080] As we have seen above, the wavelength selective router (4pr)according the present invention is useful in WDM bidirectional opticaltransmissions. It suppresses the multiple reflections, the limitingfactor of the bidirectional transmission systems and networks, andeffectively routes the counter-propagating signals. Therefore, thewavelength selective router (4pr) according the present inventionsimplifies the bidirectional signal transmissions with low cost.

[0081] While the foregoing invention has been described in terms of theembodiments discussed above, numerous variations are possible.Accordingly, modifications and changes such as those suggested above,but not limited thereto, are considered to be within the scope of thefollowing claims.

What is claimed is:
 1. A 4-port wavelength selective router comprisingfour ports (1, 2, 3, 4) with internal signal transmission paths betweenthree port pairs ((1,2), (2,3), (3,4)), and where in; when the firstoptical signal group, Group A, consisting of one or more signals withdifferent wavelengths, and the second optical signal group, Group B,consisting of one or more signals with different wavelengths excludingsignals in Group A, propagate bidirectionally, at port (2) Group Asignals are inputted and Group B signals are outputted; at port (3)Group B signals are inputted and Group A signals are outputted; betweenport (2) and (3) only Group A signals are transmitted from port (2) toport (3), and no signals can be transmitted in the other direction;between port (1) and (2), and port (3) and (4) the Group B signals orboth Group A and B signals can be transmitted, but both Group A and Bsignals are not transmitted simultaneously through the two port pairs(in other words, between port (1) and (2), and port (3) and (4) at leastone port pair transmits only the Group B signals); between port (1) and(2), and port (3) and (4) optical waves can be transmittedbidirectionally, but at least one port pair transmits the signals in onedirection, from port (1) to (2) or from port (3) to (4); and signalsinputted to a specific port (1, 2, 3, 4) cannot be transmitted to morethan one port (1, 2, 3, 4) simultaneously.
 2. A 4-port wavelengthselective router defined in claim 1, where in; Group B signals aretransmitted from Port (1) to Port (2); Group A signals are transmittedfrom Port (2) to Port (3); Group B signals are transmitted from Port (3)to Port (4); and no signals are transmitted between other port pairs andin the other direction.
 3. A 4-port wavelength selective router definedin claim 1, where in; Group B signals are transmitted from Port (1) toPort (2); Group B signals are transmitted from Port (2) to Port (1);Group A signals are transmitted from Port (2) to Port (3); Group Bsignals are transmitted from Port (3) to Port (4); and no signals aretransmitted between other port pairs and in the other direction.
 4. A4-port wavelength selective router defined in claim 1, where in; Group Bsignals are transmitted from Port (1) to Port (2); Group A signals aretransmitted from Port (2) to Port (3); Group B signals are transmittedfrom Port (3) to Port (4); Group B signals are transmitted from Port (4)to Port (3); and no signals are transmitted between other port pairs andin the other direction.
 5. A 4-port wavelength selective router definedin claim 1, where in; Group A and B signals are transmitted from Port(1) to Port (2); Group A signals are transmitted from Port (2) to Port(3); Group B signals are transmitted from Port (3) to Port (4); and nosignals are transmitted between other port pairs and in the otherdirection.
 6. A 4-port wavelength selective router defined in claim 1,where in; Group A and B signals are transmitted from Port (1) to Port(2); Group A signals are transmitted from Port (2) to Port (3); Group Bsignals are transmitted from Port (3) to Port (4); Group B signals aretransmitted from Port (4) to Port (3); and no signals are transmittedbetween other port pairs and in the other direction.
 7. A 4-portwavelength selective router defined in claim 1, where in; Group Bsignals are transmitted from Port (1) to Port (2); Group A signals aretransmitted from Port (2) to Port (3); Group A and B signals aretransmitted from Port (3) to Port (4); and no signals are transmittedbetween other port pairs and in the other direction.
 8. A 4-portwavelength selective router defined in claim 1, where in; Group Bsignals are transmitted from Port (1) to Port (2); Group B signals aretransmitted from Port (2) to Port (1); Group A signals are transmittedfrom Port (2) to Port (3); Group A and B signals are transmitted fromPort (3) to Port (4); and no signals are transmitted between other portpairs and in the other direction.
 9. A 4-port wavelength selectiverouter defined in claim 1, where in; Group B signals are transmittedfrom Port (1) to Port (2); Group A signals are transmitted from Port (2)to Port (3); Group A and B signals are transmitted from Port (3) to Port(4); Group A and B signals are transmitted from Port (4) to Port (3);and no signals are transmitted between other port pairs and in the otherdirection.
 10. A 4-port wavelength selective router with four ports (1,2, 3, 4) comprising; a first polarization splitting/combining meansseparating an optical wave into two orthogonally polarized waves, andcombining the two orthogonally polarized optical waves into an opticalwave; a first path selection means changing the optical path dependingon the polarization and the propagation direction of the optical wave; asecond path selection means changing the optical path depending on thepolarization and the propagation direction of the optical wave; a secondpolarization splitting/combining means separating an optical wave intotwo orthogonally polarized waves, and combining the two orthogonallypolarized optical waves into an optical wave; a first polarizationrotation means, located between the first polarizationsplitting/combining means and the first path selection means, changingtwo optical waves with parallel polarizations into orthogonallypolarized ones or two orthogonally polarized optical waves into oneswith parallel polarizations; a wavelength selective filter (WF), locatedbetween the first and second path selection means, selectively passingthe optical waves as their wavelengths; and a second polarizationrotation means, located between the second polarizationsplitting/combining means and the second path selection means, changingtwo optical waves with parallel polarizations into orthogonallypolarized ones or two orthogonally polarized optical waves into oneswith parallel polarizations; and where in; the first polarizationsplitting/combining means, the first path selection means, the secondpolarization splitting/combining means, and the second path selectionmeans are arranged along the optical wave propagation direction withpredetermined spacing.
 11. A 4-port wavelength selective router definedin claim 10, where in; the polarization dividing/combining meanscomprises the polarization splitters (BC1, BC4) which are arranged sothat they cause walk-offs in the same direction for the incidentextraordinary waves; the path selection means comprises the polarizationsplitter (BC2, BC3) which are arranged so that they cause walk-offs forthe incident extraordinary waves in a direction perpendicular to thewalk-off direction of the said polarization splitters (BC1, BC4); thepolarization rotation means comprises the reciprocal rotator (RR1, RR2)and the non-reciprocal rotator (FR1, FR2), arranged along thepropagation direction of the incident wave, where the reciprocal rotator(RR1, RR2) is composed of two rotators (PAR, NAR) with opposite rotationdirections; and the wavelength selective filter (WC) comprises threefilters (WF1, WF2, WF3) which are arranged along the walk-off directionof said the polarization splitter (BC2, BC3).
 12. A 4-port wavelengthselective router defined in claim 11, where in; the filter (WF2),located in the middle of the wavelength selective filter (WF; WF1, WF2,WF3), is a optical band pass filter with a pass-band and a stop-band,passes the signals within the pass-band, and blocks the signals withinthe stop-band; the other filters (WF1, WF3) are also optical band passfilters with pass-bands and stop-bands and the pass-band and thestop-band of at least one of the two filters (WF1, WF3) should coincidewith the stop-band and the pass-band of the said filter (WF2),respectively.
 13. A 4-port wavelength selective router defined in claim11, where in; the filter (WF2), located in the middle of the wavelengthselective filter (WF; WF1, WF2, WF3), is a optical comb filter withperiodic pass-bands and stop-bands, passes the signals within thepass-bands, and blocks the signals within the stop-bands; the otherfilters (WF1, WF3) are also optical comb filters with pass-bands and astop-bands and the pass-bands and the stop-bands of at least one of thetwo filters (WF1, WF3) should coincide with the stop-bands and thepass-bands of the said filter (WF2), respectively.
 14. A 4-portwavelength selective router with four ports (1, 2, 3, 4) comprising; anoptical circulator (Cir) with an input port (a), an output port (c), anda common port (b); and a wavelength selective coupler (WSC) with twoinput/output ports (d, f) and a common port (e); and where in; the inputport (a) of the circulator (Cir) is connected to port (1); the commonport (b) of the circulator (Cir) is connected to port (2); the outputport (c) of the circulator (Cir) is connected to one of the input/outputport (d) of the wavelength selective coupler (WSC); the common port (e)of the wavelength selective coupler (WSC) is connected to port (3); andthe other input/output port (f) of the wavelength selective coupler(WSC) is connected to port (4).
 15. A 4-port wavelength selective routerwith four ports (1, 2, 3, 4) comprising; two wavelength selectivecouplers (WSC, WSC1) with two input/output ports ((d, f), (d′, f′)) anda common port (e, e′); and two optical isolators (Iso1, Iso2) with aninput port (g, g′) and an output port (h, h′); and where in; the inputport (g′) of the optical isolator (Iso2) is connected to port (1); theoutput port (h′) of the isolator (Iso2) is connected to one of theinput/output port (f′) of the wavelength selective coupler (WSC1); thecommon port (e′) of the wavelength selective coupler (WSC1) is connectedto port (2); the other input/output port (d′) is connected to the inputport (g) of the other optical isolator (Iso1); the output port (h) ofthe isolator (Iso1) is connected to one of the input/output ports (d) ofthe other wavelength selective coupler (WSC); the common port (e) of thewavelength selective coupler (WSC) is connected to port (3); and theother input/output port (f) of the wavelength selective coupler (WSC) isconnected to port (4).
 16. A 4-port wavelength selective router withfour ports (1, 2, 3, 4) comprising; two wavelength selective couplers(WSC, WSC1) with two input/output ports ((d, f), (d′, f′)) and a commonport (e, e′); and two optical isolators (Iso1, Iso2) with an input port(g, g′) and an output port (h, h′); and where in; the input/output port(f′) of the wavelength selective coupler (WSC1) is connected to port(1); the common port (e′) of the wavelength selective coupler (WSC1) isconnected to port (2); the input/output port (d′) of the wavelengthselective coupler (WSC1) is connected to the input port (g) of theoptical isolator (Iso1); the output port (h) of the isolator (Iso1) isconnected to port (d) of the wavelength selective coupler (WSC); thecommon port (e) of the wavelength selective coupler (WSC) is connectedto port (3); the input/output port (f) of the wavelength selectivecoupler (WSC) is connected to input port (g′) of the other isolator(Iso2); and the output port (h′) of the isolator (Iso2) is connected toport (4).
 17. A 4-port wavelength selective router with four ports (1,2, 3, 4) comprising; a wavelength selective coupler (WSC1) with twoinput/output ports (d′, f′) and a common port (e′); and an opticalcirculator (Cir1) with an input port (a′), an output port (c′), and acommon port (b′); and where in; one of the input/output ports (f′) ofthe wavelength selective coupler (WSC1) is connected to port (1); thecommon port (e′) of the wavelength selective coupler (WSC1) is connectedto port (2); the other input/output port (d′) of the wavelengthselective coupler (WSC1) is connected to the input port (a′) of theoptical circulator (Cir1); the common port (b′) of the circulator (Cir1)is connected to port (3); and the output port (c′) of the opticalcirculator (Cir1) is connected to port (4).
 18. A 4-port wavelengthselective router defined in one of claim 14˜claim 17, where in; thewavelength selective coupler (WSC, WSC1) is a wavelength-divisionmultiplexer routing two groups of optical signal in different wavelengthbands inputted at the two input/output ports ((d, f), (d′, f′)) to thecommon port (e, e′), or routing the optical signals inputted at thecommon port (e, e′) to the two input/output ports ((d, f), (d′, f′)) astheir wavelengths.
 19. A 4-port wavelength selective router defined inone of claim 14˜claim 17, where in; the wavelength selective coupler(WSC, WSC1) is a wavelength-interleaver routing two adjacent opticalsignals inputted at the two input/output ports ((d, f), (d′, f′)) to thecommon port (e, e′), or routing the optical signals inputted at thecommon port (e, e′) to the two input/output ports ((d, f), (d′, f′)).20. A 4-port wavelength selective router with four ports (1, 2, 3, 4)comprising; two optical circulators (Cir, Cir1) with an input port (a,a′), an output port (c, c′) and an common port (b, b′); and two opticalfilters (Fil1, Fil2), with an input port (j, j′) and an output port (k,k′), having different pass-band and stop-band; and where in; the inputport (a) of the circulator (Cir) is connected to port (1); the commonport (b) of the circulator (Cir) is connected to port (2); the outputport (c) of the circulator (Cir) is connected to the input port (j) ofthe filter (Fil1); the output port (k) of the filter (Fil1) is connectedto the input port (a′) of the other circulator (Cir1); the common port(b′) of the circulator (Cir1) is connected to port (3); the output port(c′) of the circulator (Cir1) is connected to the input port (j′) of thefilter (Fil2); and the output port (k′) of the filter (Fil2) isconnected to port (4).
 21. A 4-port wavelength selective router definedin claim 20, where in; the optical filters (Fil1, Fil2) are optical bandpass filters passing/blocking the optical signals within thepass-band/stop-band, and the pass-band and the stop-band of one of twothe filters (Fil1, Fil2) are opposite to those of the other filter. 22.A 4-port wavelength selective router defined in claim 20, where in; theoptical filters (Fil1, Fil2) are optical comb filters having periodicpass-bands/stop-bands and passing/blocking within the signals within thepass-bands/stop-bands, and the pass-bands and the stop-bands of one oftwo the filters (Fil1, Fil2) are opposite to those of the other filter.23. A 4-port wavelength selective router defined in claim 1, where in;with the connection between the port (1) and port (4), the opticalsignals propagating from port (2) to port (3) can not be transmittedfrom port (3) to port (2) and the optical waves propagating from port(3) to port (2) can not be transmitted from port (2) to port (3).
 24. Abidirectional add/drop multiplexer, using a 4-port wavelength selectiverouter defined in claim 1, comprising; a de-multiplexer whose input portis connected to port (4) of the 4-port wavelength selective routerdefined in claim 1; a multiplexer whose output port is connected port(1) of the 4-port wavelength selective router defined in claim 1; one ormore 2×2 optical switches (Sw) connected between the output ports of thede-multiplexer and the input ports of the multiplexer; one or morereceivers (RX) connected to one of the ports of the 2×2 switches (Sw);and one or more transmitters (TX) connected to another of the ports ofthe 2×2 switches (Sw).